objective c - Calculating the max value for a core data attribute - NSCFNumber error

Question

I need to find out the maximum value of an attribute of a core data entity.

I'm still firmly in the Cocoa learning curve, and this is a simple test app that I'm using to learn.

The app imports fortunes from a text file and displays a table on the screen. The imports are done in a separate, background thread.

I found this code online, which I have attempted to get working:

- (double)getMaxID  
{  
    NSLog(@"in getMaxID");  // debug  

    // Use a new moc with the original persistentStoreCoordinator to ensure thread safety  
    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];  
    [moc setPersistentStoreCoordinator:[[self delegate] persistentStoreCoordinator]];  

    // Create fetch  
    NSFetchRequest *fetch = [[NSFetchRequest alloc] init];  
    [fetch setEntity:[NSEntityDescription entityForName:@"Fortune"   inManagedObjectContext:moc]];  
    [fetch setResultType:NSDictionaryResultType];  

    // Expression for Max ID  
    NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"id"];  
    NSExpression *minExpression = [NSExpression expressionForFunction:@"max:" arguments:  [NSArray arrayWithObject:keyPathExpression]];  
    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];  
    [expressionDescription setName:@"maxID"];  
    [expressionDescription setExpression:minExpression];  
    [expressionDescription setExpressionResultType:NSDoubleAttributeType];  
    [fetch setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];  

    // Execute the fetch.  
    double theID = 0;  
    NSError *error = nil;  
    NSArray *objects = nil;  
    objects = [moc executeFetchRequest:fetch error:&error];  // crashes here  

    if (objects && ([objects count] > 0))  
    {  
        NSLog(@"query successful"); // debug  
        theID = [((NSNumber *)[[objects objectAtIndex:0] valueForKey:@"maxID"]) doubleValue];  
    }  
    else  
    {  
        NSLog(@"Setting default value for theID"); // debug  
        theID = 0;  
    }  

    return(theID);  
} 

My entity is called "Fortune" and the attribute is called "id" (a Double).

When the code runs it crashes when the fetch request is executed. The console shows this:

2009-12-18 00:53:42.777 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10  
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] An uncaught exception was raised  
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10  
2009-12-18 00:53:42.779 FortunesHelperMVC[4027:1703] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10'  
*** Call stack at first throw:  
(
0 CoreFoundation 0x00007fff83ed9444 __exceptionPreprocess + 180  
1 libobjc.A.dylib 0x00007fff85fbb0f3 objc_exception_throw + 45  
2 CoreFoundation 0x00007fff83f321c0 +[NSObject(NSObject) doesNotRecognizeSelector:] + 0  
3 CoreFoundation 0x00007fff83eac08f ___forwarding___ + 751  
4 CoreFoundation 0x00007fff83ea81d8 _CF_forwarding_prep_0 + 232  
5 Foundation 0x00007fff88a5609e +[_NSPredicateUtilities max:] + 46  
6 Foundation 0x00007fff8893ce72 -[NSFunctionExpression expressionValueWithObject:context:] + 530  
7 CoreData 0x00007fff8613b5b1 -[NSMappedObjectStore executeFetchRequest:withContext:] + 2081  
8 CoreData 0x00007fff8613ad10 -[NSMappedObjectStore executeRequest:withContext:] + 80  
9 CoreData 0x00007fff86108900 -[NSPersistentStoreCoordinator(_NSInternalMethods) executeRequest:withContext:] + 688  
10 CoreData 0x00007fff8610621b -[NSManagedObjectContext executeFetchRequest:error:] + 267  
11 FortunesHelperMVC 0x0000000100001d9d -[ImportOperation getMaxID] + 572  
12 FortunesHelperMVC 0x0000000100001f95 -[ImportOperation main] + 330  
13 Foundation 0x00007fff888f406d -[__NSOperationInternal start] + 681  
14 Foundation 0x00007fff888f3d23 ____startOperations_block_invoke_2 + 99  
15 libSystem.B.dylib 0x00007fff86a98ce8 _dispatch_call_block_and_release + 15  
16 libSystem.B.dylib 0x00007fff86a77279 _dispatch_worker_thread2 + 231  
17 libSystem.B.dylib 0x00007fff86a76bb8 _pthread_wqthread + 353  
18 libSystem.B.dylib 0x00007fff86a76a55 start_wqthread + 13  
)  
terminate called after throwing an instance of 'NSException'  

Any ideas why this isn't working? I'm stumped after a lot of Googling.

Thanks

Darren.

Answer 1

I had to solve a similar problem and approached it slightly differently by:

• querying the entity
• ordering by the attribute you want
• limit the fetch results to 1

The code for this is below. I am querying an entity called 'entityName' and retrieving the max value for the attribute 'sequenceId'.

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *res = [NSEntityDescription entityForName:@"entityName" inManagedObjectContext:managedObjectContext];
[request setEntity:res];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"sequenceId" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];

[request setFetchLimit:1];

NSError *error = nil;
NSArray *results = [managedObjectContext executeFetchRequest:request error:&error];
[request release];
if (results == nil) {
    NSLog(@"error fetching the results: %@",error);
}

NSInteger maximumValue = 0;
if (results.count == 1) {
    Result *result = (Result*)[results objectAtIndex:0];
    maximumValue =  [result.sequenceId integerValue];
}